The game has changed; and with elections across much of Britain this week, the timing for the Conservatives could scarcely be worse. YouGov’s latest poll for the Sunday Times puts them at 29%. It’s the first time since 2004 that they have slipped below 30%.That’s not all. For the first time since he became Prime Minister, David Cameron risks not just a short-term setback but a long-term nightmare.

Until last month, the Conservatives had reason to stay cheerful. Despite faltering living standards and rising unemployment, the party’s support remained at roughly its 2010 election level, of 37%. But in the five weeks since the Budget, it has lost one fifth of its support. By the middle of last week it was already down to 32%. In the wake of news that Britain’s economy is back in recession, and the controversy over Jeremy Hunt’s position, it has slipped below 30% for the first time since 2004.

Worse still, the Prime Minister is no longer a clear asset to his party. Immediately before the Budget, 44% thought he was doing well, while 49% thought he was doing badly. His net rating, minus 5, was pretty good for a Prime Minister in mid-term. Now his rating is minus 31 (well 32%, badly 63%). Only Gordon Brown can match such a collapse in popularity, when he scrapped plans to hold a snap election in October 2007. It is not a happy precedent.

Cameron can console himself with two crumbs of comfort. The first is that the slump in his fortunes might be short-lived. A period of economic calm and governmental competence might erase memories of the past few torrid weeks. The second is that voters’ fury with the Tories has not so far converted into a passion for Labour. Ed Milband remains less popular than Cameron (albeit by a rapidly narrowing margin), and Cameron and George Osborne are still reckoned to be better custodians of the economy than Miliband and Ed Balls.

Against that possible upside for the Tories is the danger of a terrifying downside. The party that is making the biggest gains is the UK Independence Party. At 10% its support is its highest ever. As many as 1.4 million people who voted Conservative two years ago have transferred their allegiance to UKIP.

This could be just a fleeting protest. Past Tory governments have shed votes in huge numbers in mid-term to the Liberal Democrats, only to recover most of them at the next election. With the Lib Dems now in government, UKIP is the obvious temporary receptacle for unhappy Tories who don’t want to vote Labour. Maybe they will return home by 2015 when voters have to choose a government.

Yet here’s an alternative scenario. Suppose the economy continues in the doldrums. Suppose Cameron’s ratings stay on the floor. In 2014, a year before the next general election, Europe’s voters will decide who represents them in the European Parliament. Voting will take place under a proportional system that helps smaller parties – and the anti-EU UKIP most of all. Last time, in 2009, it came second, ahead of Labour. Unless the Tories recover, I would not bet heavily against UKIP topping the poll in 2014, or coming close.

If that happens, the dynamic of the next general election could crucify the Tories. UKIP would have the credibility it has always craved. Under our first-past-the-post system for electing MPs, it might end up with too few votes to win many seats for itself – but quite enough to scupper the Tories. Suppose it wins over just 2,000-3,000 unhappy Tories in each of the key marginals. This kind of division on the Right would be enough to cost Cameron up to thirty seats, and hand victory to Ed Miliband. If the shift to UKIP is much greater, Labour could win by a landslide.

If that sounds far-fetched, a similar process helped the Tories and crippled Labour in the Eighties. Then, the short-lived Social Democratic Party divided the left-of-centre vote and handed big victories to Margaret Thatcher.

Historically, one of the Conservatives’ huge advantages has been that it has, for all practical purposes, monopolised the right-of-centre vote. Unwittingly, Cameron may be jeopardising this huge advantage. By going into coalition with Nick Clegg, he has stripped the Lib Dems of their appeal as vehicle for protest. If, as a result, UKIP establishes itself as the preferred destination for discontented Tories and divides the right-of-centre vote, then the Conservatives’ ability to win future elections could be fatally damaged.